Explicit per-table PG stats

@remh this is for 4.1. I see this is new beginnings more than a definitive PG check.

I have spent some time reworking the check to make the collection of per-table stats easier. There are now up to 3 statistics queries executed: one for all databases, one for all listed tables (or relations in pg parlance) and one for all indices of the listed tables.

Tagging is per database, per table.

There is a number of other metrics we can get. It should be relatively easy to add without altering the structure of the check (in particular the collect_stats part that parses results).

The table list is made explicit to not generate boat loads of useless metrics.

@clofresh probably of interest to you too.